Old Boy, by himself, note, unfashionable (8)
I believe the answer is:
obsolete
'unfashionable' is the definition.
(obsolete means out of date)
'old boy by himself note' is the wordplay.
'old boy' becomes 'OB' (abbreviation for a private school 'old boy').
'by himself' becomes 'sole' (on one's own).
'note' becomes 'te' (musical note).
'ob'+'sole'+'te'='OBSOLETE'
(Other definitions for obsolete that I've seen before include "Gone out of use and no longer effective" , "No longer useful" , "Out-of-date" , "Disused" , "Outdated" .)